Here's why:
* Common nouns refer to general categories of people, places, or things.
* Proper nouns refer to specific people, places, or things.
"Ivory" refers to a general material, not a specific item. It could be ivory from any elephant tusk or any other source.
Examples:
* Common noun: The ivory statue was beautiful.
* Proper noun: The Taj Mahal is a beautiful building. (Taj Mahal is a specific building)